Causes

Returns to service are caused by a failure of mating or of pregnancy. Lack of libido in the boar, reduced receptivity in the sow, physical factors such as penis damage or vasectomy of faulty insemination technique all fail to deliver sperm to the female reproductive tract and cause returns to service at 18-24 days.

Failure of ovulation or of the eggs to reach the uterus as a result of adhesions, uterine infection, with organisms such as Trueperella pyogenes, poor quality semen from the boar or at insemination may result in failure of fertilisation. The reproductive tract of the sow then behaves as if mating had never occurred and the next heat develops normally at 18-24 days. Where returns to service occur at multiples of 18-24 days (at 48 days) then it is likely that a regular heat period has been missed.

Where fertilisation has taken place and the embryos die or fail to implant at 12-15 days after service due to infection, mixing or heat stress, returns to service occur at irregular intervals. Where older products of conception die and are reabsorbed (before 35 days of gestation) or are unseen due to infections such as parvovirus, then irregular returns to oestrus also occur.

Mode of transmission

Returns to service may result from infectious disease such as leptospirosis or parvovirus infection. Leptospiral infection may result in repeated returns as infection is not eliminated from the uterus but solid immunity develops in parvovirus infection and returns are restricted to a wave of reproductive problems following the infection of a naive farm which then become restricted to gilts or susceptible stock entering the breeding pool. Transmission of these infections may be oronasal or venereal.

Clinical signs

These consist of observation of oestrus in pigs thought to have been mated or inseminated successfully. They may be regular (at 18-24 day intervals) or multiples of 18-24 days or irregular (30 or 35 days, etc.).

Successful observation requires identification of the sow, recording of oestrus, recording of mating, regular examination for returns at 18-24 days post service or insemination which may include observation of the vulva, detection of oestrous behaviour, testing with the boar or testing for willingness to stand.

There may be evidence of mating to suggest that it was unsatisfactory or evidence for infection such as vulval discharge following mating. Clinical signs of diseases such as erysipelas which can affect implantation and stimulate irregular returns may be observed. Regular returns to service occur normally at levels of 5-9%, but levels higher than these are abnormal. Similarly 2-4% irregular returns commonly occur and higher levels of these are also abnormal.

Abnormal levels of return to service can be established, provided that sows are identified individually, oestrus and mating/insemination dates are recorded and mating animals are closely observed from18-24 days post mating and daily thereafter until pregnancy is confirmed.

Regular inspection, exposure to contact with a boar and testing of animals suspected of returning using a boar or back pressure help confirm oestrus. Regular returns at 18-24 days suggest failure of fertilisation before 10 days' gestation and returns at multiples of 18-24 days suggest missed heats. There may be failure to mate during unsupervised service and observation or physical examination of boars may confirm this. Where service was recorded, the involvement of a particular boar or overuse may be identified. Semen from boars or insemination packs should be examined and can contain low sperm numbers, poor sperm morphology or poor sperm motility.

Heat stress may cause regular returns. The cause of irregular returns resulting from the death of the products of conception can sometimes be identified by a the presence of vulval discharge in local infection, by post-mortem examination of uteri or by testing for specific bacteria and viruses such as leptospirosis and parvovirus and identification of erysipelas and other generalised infections.

Postmortem lesions

Post-mortem examination of the affected animals is rarely carried out, and if so, usually at slaughter. Semen examination confirms whether or not a boar is potentially fertile, but pathological examinations can identify problems such as spinal or joint injuries which reduce the efficacy of service.

Testicular abnormalities can also be detected. The female reproductive tract can also be examined and the patency of the vulva, vagina and cervix recorded. Uterine abnormalities which would prevent implantation may be detected and the presence of any degenerating embryos or remnants of previous pregnancies detected. Blockage of the fallopian tubes or fibrous bands around the ovary may prevent the discharge of eggs. The ovary itself should appear normal with corpora lutea and developing follicles.

Treatment and prevention

When regular returns to service are identified, the sows concerned should be re-served immediately using fertile boars of good libido and of the appropriate weight or inseminated with freshly collected semen or semen from reliable suppliers kept in optimum conditions. Where overuse of boars might occur, artificial insemination can be used as a supplement or instead of a second service.

Service should take place in the cool morning or in shady water-cooled houses if heat stress is the cause of the regular returns. Sows returning for a third time should be culled unless the herd inventory must be maintained. Unreliable boars and those with poor quality semen should be culled.

Local uterine infections can be treated by injection with a broad spectrum antimicrobial, such as tetracycline or ampicillin and boars associated with such infections should have their prepuces washed and antimicrobial instilled. Specific diseases such as erysipelas can be treated. Longer term control may require vaccination against generalised disease such as erysipelas, and against parvovirus, leptospirosis and other reproductive tract diseases. Management factors such as lighting, improved exposure to boar contact and improved nutrition may all result in better observation or oestrus and reduce long term regular returns to service.